Your attempt only says that you think that if AC is fed into the dynamo output it will become a motor. That is no more than guess. What is your reason for thinking this will work? As you ask, **how** does this happen?

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The dynamo produces AC output, so in theory yes, it should work in reverse if AC is input : see this demonstration.

The difficulty with using mains AC is that the input frequency (50Hz) may be too high for the rotor, at least initially. If the rotor requires a high torque to get it started, it will move only a fraction of a turn before the reverse cycle of current starts to push it the other way. In such circumstances the rotor will not start moving, or will just oscillate or hum while the wires get hot.

However, if the rotor is set spinning at about 50Hz then it will be able to get in synch with the AC input and continue working. Prior to that, or if the mechanical load on it varies, the rotor movement may be jerky.

The input frequency of AC and the rotational frequency of the rotor must match fairly closely. As the frequency gets higher, the narrower is the window of tolerance, and the more likely they are to get out of synch, resulting in jerky motion, or the rotor stopping altogether.

The reason why the hand-cranked generator/motor combination works so well in the demo is that the frequency of the output from the generator is well-matched to that of the motor, because the two devices are identical, and because the AC frequency starts very low and builds up, allowing the motor time to catch up.